Water-cooled cholesteric liquid crystal infrared imaging device

ABSTRACT

An infrared imaging device for the detection and measurement of hazardous irradiances in a laser beam caused by atmospheric scintillation. The 10.6 micrometer field is displayed visibly through the use of cholesteric liquid crystals on a water-cooled mylar film.

United States Patent Horton 1 Mar. 27, 1973 [54] WATER-COOLEDCHOLESTERIC LIQUID CRYSTAL INFRARED IMAGING DEVICE Richard F. Horton,Fredericksburg, Va.

The United States of America as represented by the Secretary of the NavyFiled: June 11, 1971 Appl. No.: 152,306

Inventor:

Assignee:

U.s. Cl ..2so/s3.3 11, 250/833 HP rm. Cl "0011 1/16 Field of Search250/83 R, 83 .3 11,333 R, 250/833 HP UNITED STATES PATENTS 3,410,99911/1968 Fergason et al 250/833 R 3,569,709 3/1971 Wank ..250/83 R3,604,930 9/1971 Allen ..2S0/83.3 HP X Primary Examiner-James W.Lawrence Assistant Examiner-Davis L. Willis Attorney-R. S. Sciascia etal.

[57] ABSTRACT An infrared imaging device for the detection andmeasurement of hazardous irradiances in a laser beam caused byatmospheric scintillation. The 10.6

micfometeriield is displayed visibly throngh the use of cholestericliquid crystals on a water-cooled mylar film.

9 Claims, 10 Drawing Figures PATENTEDMARZ? m5 3, 723. 739

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ATTORNEY PATENTEDHARZYISIS 3,723,739

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SHEET U UF 4 INDICATED IRRADIANCE RELATIVE v IRRADIANCE PROFILE /e T T HTIME (T) r RADIAL DISTANCE FROM CENTER or GAUSSIAN COLOR -($CALE NOTLINEAR IN WAVE- LENGTH TO ALLOW STRAIGHT LINE PLO VIOLET 1 n I l I I I II I 0 I00 200 mw/cm IRRADIANCE WATER-COOLED CHOLESTERIC LIQUID CRYSTALINFRARED IMAGING DEVICE BACKGROUND OF THE INVENTION l. Field of theInvention The present invention relates generally to a device fordetecting and measuring hazardous irradiances caused byatmosphericscintillation in a propagating CO laser beam.

2. Description of the Prior Art A means of directly observingatmospheric scintillation in a propagating CO laser beam is desirable.Information about irradiance fluctuations at this wave length isnecessary for safety studies. These fluctuations may produce irradiancelevels which are considered unsafe for human exposure. Irradiancesgreater than 100 mW/cm may produce lesions in the skin and cornea.

Atmospheric scintillation is a phenomenon associated with radiationpropagating through the atmosphere. Spatial and temporal variations inthe index of refraction of the air give rise to random focusing anddefocusing of the propagating radiation. This is observed asfluctuations in the irradiance at points within the propagatingradiation. The inventive water-cooled, cholesteric liquid crystal,infrared imaging device is capable of observing atmosphericscintillation at a wave length of micrometers.

Liquid crystals are a state of matter characterized by molecularordering intermediate between the randomness of a liquid and theordering of a crystalline solid. This is a result of weak intermolecularforces which partially order the large organic molecules. This orderingoccurs in small domains. At the same time, the flow and viscosity aretypical of liquids. These characteristics are responsible for the name,liquid crystals.

Liquid crystal materials are composed of elongated organic molecules. Inthe cholesteric state, layers are formed in which the long axis of thesemolecules are parallel. In each domain, these molecular layers arestacked, one upon another with the molecular axes direction in eachsucceeding layer rotated some constant amount with respect to themolecular axes direction in the preceeding layer. Thus, along adirection perpendicular to the layers the molecular axes describe ahelix as shown in FIG. 1. FIG. 1 illustrates an idealized liquid crystalstructure showing helical molecular order. The arrows in FIG. 1 indicatethe long axes of molecules.

When the pitch of the helix is equal to the wavelength of lighttraversing the medium in the direction of the helical axis, thecircularly polarized component of this light whose electrical vectorrotates with the same sense as the helix, is sharply attenuated andreflected or scattered-back. If white light is incident on liquidcrystals, the wavelength of the scattered light is determined by thehelical pitch. For a given helical pitch, the liquid crystals appear tobe a certain color.

The pitch of the helix of certain cholesterical liquid crystals, andtherefore the apparent color, is a strong function of temperature over arange of a few degrees. The liquid crystals used in a specific operatingembodiment of the inventive infrared imaging device had a visible colorrange of about 2 C when illuminated with white light at normalincidence. At 24 C, the liquid crystals first begin to show redcoloring. Below 24 C, visible light was not scattered and the liquidcrystals were colorless. Violet light was scattered when the temperaturewas 26 C. Temperatures above 26 C caused the liquid crystals again to becolorless.

Cholesteric liquid crystals have been used in the past to displayinfrared fields. In each case, the infrared radiation was absorbedcausing a temperature rise which was visibly displayed by the colors ofthe liquid crystals. The temperature rise was assumed to be proportionalto the irradiance, so that colors displayed corresponded to theirradiances of the field.

SUMMARY OF THE INVENTION INVENTION The inventive device provides a newmeans for obtaining information about irradiance fluctuations caused byatmospheric scintillation in a propagating CO laser beam. The devicemakes use of liquid crystals applied to a mylar film to enable visualobservation and measurement of the hazardous irradiances. The devicefurther includes a cooling-water reservoir to provide sufficientcooling-water to permit extended operation at high power averageirradiances.

OBJECTS OF THE INVENTION The main object of the present invention is theprovision of means for detecting and measuring hazardous irradiancesproduced by C0 lasers.

Other objects, advantages and novel features of the present inventionwill become apparent from the following detailed description of theinvention and considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an illustration of anidealized liquid crystal structure showing helical molecular order.

FIG. 2 is a front view of a preferred embodiment of the instantinvention.

FIG. 3 is a side view of a preferred embodiment of the instantinvention.

FIG. 4 is a schematic diagram of a laboratory setup used to determinethe time constant, the resolution and the operating range of theinventive imaging device.

FIG. 5 is a plot of the angular beam profile at 3.0 meters from focus.

FIG. 6 is a circuit diagram of the delay and strobe unit utilized in thecircuit of FIG. 4.

FIG. 7 is a graph showing the color response of the inventive detectorin response to the center of the Gaussian beam pulse.

FIG. 8 is a graph showing the decay of the indicated irradiance withtime.

FIG. 9 is a plot of a relative irradiance of the beam versus the radialdistance from the center of the beam, showing the radius (r)corresponding to I /e.

FIG. 10 is a graph showing the color response of the inventive imagingdevice versus irradiance.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 is a front viewof a preferred embodiment of the instant invention with the mylar film 3(FIG. 3) removed so that the impellor cavity 14 may be observed. Theimpellor revolves in the impellor cavity 14 (FIG. 3). Holes 16 provideoutlets for the coolingwater.

FIG. 3 is a side view of a preferred embodiment of the inventive imagingdevice. The major portion of the device is the cooling-water reservoir 1which provides sufficient cooling-water to allow several minutesoperation'at very high power/area average irradiances. The drive shaft15 connects the drive unit to the impellor 5 in the impellor cavity 14.Any suitable drive unit may be used, for example, a small motor may becoupled to the drive shaft 15 through a reduction pulley system. Theimpellor cavity 14 is formed between the front wall of thewaterreservoir 1 and the mylar film 3. The distance between the mylar film 3and the impellor 5 may be varied by changing the thrust washers 11between the impellor 5 and the front wall of the water reservoir 1.

Water drawn from the reservoir moves through the center of the shaftthrough the water inlets 12 into the impellor cavity 14 where it isaccellerated radially outward between the impellor 5 and the mylar film3. Besides acting as a mechanism moving a bulk of water, impellor 5 actsto provide a turbulance region 6 at the back side of the mylar film 3which expedites heat transfer. Water returns to the reservoir throughholes 13 in the wall at the outer radius of the impellor. The holes 13are only in the lower half of the impellor cavity 14 so that a waterhead 4 may be maintained in the upper half of the impellor cavity. Thewater level in the water reservoir 7 should be maintained at asufficient level to provide an adequate supply of cooling-water by thedevice. A front bearing 8 is provided between the drive shaft 15 and thefront wall of the water reservoir 1. Further, a rear bearing and seal 17is provided between the rear wall of the water reservoir 1 and the driveshaft 15.

Several different thicknesses of mylar film may be used. The thicknessesused in a prototype of the invention ranged in size from 0.5 to 5 mils.The mylar film which worked most satisfactorily had a thickness of 3mils. The mylar film was heated after insertion in the device. Thiscaused the film to become taut, providing a more workable surface.

A reflective coating of aluminum was deposited on the back surface ofthe mylar film to prevent infrared radiation, which was not absorbed bythe film, from being transmitted to the water. Absorption of thisradiation by the water would cause unwanted elevation of the watertemperature. The reflected radiation could be further absorbed by themylar film. The liquid crystals should be applied to the mylar film whenthe water temperature is around 40 C. This facilitates the evaporationof the solvent used with the cholesteric liquid crystals. After thewater temperature was lowered to the operating region, from 24 to 26 C,the liquid crystals should be brushed lightly with a camel's hair brush.This increases the brilliance of the colors displayed by the liquidcrystals.

FIG. 4 illustrates a laboratory setup that was used to determine thetime constant, resolution and operating range of a specific operatingembodiment of the inventive imaging device. A C0, laser was used toprovide the micrometer infrared radiation. It produced an approximatelyGaussian shaped beam (TEM mode) of 25 watts continuous power or asimilarly inlcnsc long-pulsed beam of H of a second duration. Thelaser's beam current, which provides the plasma with power for lasing,was monitored and displayed on the oscilloscope. The beam splitter andthe power meter were used to monitor output power of the laser. An 18inch focal length mirror was used to expand the beam providingirradiances in the l00mW/cm region. The CW beam profile was measuredwith a thermopile detector. The relative profile of the beam is shown inFIG.

suitable amounts of hot or cold water. The thermocoupic was comparedwith an alcohol thermometer at the limits of this 1.5 C range. Duringthe operation of the device, the output signal of the temperaturemonitor was displayed on one channel of the oscilloscope. The beamcurrent of the laser was displayed on the other channel. An oscilloscopecamera was used to record this data.

An additional oscilloscope camera was used for photographing the colorresponse of the liquid crystals, using Polaroid color film. Variationsof the color films response was minimized by using rolls of film fromthe same package and closely following the manufacturers recommendeddevelopment times and by maintaining the angles of illumination andobservation fixed.

The delay and strobe unit was used in determining the time constant ofthe device. This unit was used with a laser pulse rate of l per secondand with a pulse dura tion of 1/20 second. The delay and strobe circuitsstarted a timer upon sensing the laser pulse. The delay fired the strobeafter a predetermined interval of from 0.05 to 1.0 seconds. The strobeilluminated the liquid crystals so that momentary color response couldbe photographed. The firing of the strobe appeared in the oscilloscopetrace of the beam current. Delay times were measurable from thisphotographic record. A circuit diagram of the delay and strobe unitsused is shown in FIG. 6.

The delay and strobe unit was designed to facilitate the data taking bythe experimental measurement of the time constant of a liquid crystalimaging device. The circuit was designed to accept a signal from thelaser, initiate the timer and finally fire the strobe. A signal of 1.5volts is sensed from the beam current monitor of the laser by thesilicon controlled rectifier 61, allowing current to flow into thetiming circuit. The timing circuit is based upon the characteristics ofthe unijunction transistor 62. After the appropriate time interval,determined by the variable resistor 63, typically a 0-1 megohm variableresistor, the timing circuit fires the silicon controlled rectifier 64.When this fires, the induced high voltage in the secondary of theautomotive ignition coil ionizes the FT30 flash tube and the energy inthe capacitor 66, typically a microfarad capacitor, is dischargedthrough the flash tube. The ringing of the circuit reverse biases bothSCRs, returning them to the nonconductive state. A typical repetitionrate of 1 Hz is sufficient to allow recharging of the capacitor 66. Thelight output of the flash tube was such that for a 1 foot distancebetween the lamp and the device, Polaroid color film (ASA 75) was shotat f/Z.

Any radiation detecting device takes some period of time to respond to achange in the irradiance which it measures. The time constant is ameasure of this time period. Typically, the dynamic response ofdetectors to a step function in irradiance can be approximated by asingle exponent of the form [1 exp(T/t)] in which T is the time and t isthe time constant.

The response of the water-cooled, liquid crystal imaging detector isbelieved to be best described by two time constants, t, for anincreasing step in irradiance and t for a decreasing step in irradiance.It is reasonable that 2,, is greater than t,. If the rate of coolingwere greater than the rate of heating, no temperature rise could bedeveloped. Furthermore, the heat is produced by absorption through themylar, while the heat is lost to the water by conduction, the slowerprocess of the two. It is assumed that the conduction is the dominantheat loss process. The dynamic response of the device is limited by thelargest time constant, r,,- t,, was deduced from pictures taken of thecolor response to an approximately Gaussian shaped beam of 1/20 secondduration. The pictures were taken using the delay and strobe unitmentioned previously. The colors of the detector in response to thecenter of the beam are plottedin FIG. 7. It should be noted that theindicated irradiance is rising even after 0.05 seconds when the pulsestopped. The peak of the indicated irradiance occurred at 0.1 seconds.It is assumed that the decay is independent of the means by which thedevice is caused to indicate an irradiance, provided this means has notdegraded the performance of the apparatus. If this decay is exponential,the color of the detector at 0.1 seconds t,, indicates an irradiancewhich is a factor of He times the irradiance indicated at 0.1 seconds,as illustrated in FIG. 8. Let the violet color at T 0.1 secondscorrespond to a indicated irradiance of I, at the center of the beam.The radial distance corresponding to a relative irradiance of He wasfound from the plot of the beam shape, FIG. 9. Using this radialdistance, the color corresponding to an indicated irradiance of I e wasfound from a photograph taken of the device at t+0. 1. FIG. 7 indicatedthe color occurred in the decay process at 0.5 seconds. The intervalcorresponding to r is 0.4 seconds. The error due to uncertainties incolor differentiation is estimated to be :t 0.1 seconds.

The resolution of an imaging device is directly related to the abilityto resolve changes in irradiances which occur over very small distancesat the imaging surface. Usually, an alternating pattern of irradiance isused to determine this resolution. When the device being tested can justshow the alternating pattern, the spacial frequency of the pattern isquoted as resolution, typically in line pairs per millimeter.

The resolution of the inventive device was determined in a novelfashion. A Fresnel diffraction pattern was used to provide fringes ofalternating irradiances. The Fresnel diffraction pattern was produced bya nee dle placed between the focus of the mirror and the detector. Thedistance from the center of the diffraction pattern to the point atwhich the pattern could just be resolved was measured. From optical andgeometric considerations, the spacing of the lines in the pattern atthis point was calculated. This spacing is taken as the resolution ofthe device. The calculated value was 3.2 line pairs per millimeter.

The inventive liquid crystal imaging device can display 10 micrometerradiation over only a particular range of irradiances. This range isdetermined by the maximum irradiance which can be displayed and theminimum irradiance which can be detected. When the prototype imagingdevice was thermally biased to just begin red coloring with no infraredfield, a violet color was produced at a constant irradiance of 200mW/cmIf the temperature of the mylar film is assumed to be linear withrespect to irradiance, the graph depicted in FIG. 10 describes theresponse of the device. A minimum observable signal would correspond toa minimum observable change of colors. Since this is clearly asubjective value, it can only be estimated at 20mW/cm Obviously, manymodification and variations of the present invention are possible inlight of the above teachings. For instance, since the most outstandingcharacteristic of the inventive imaging device is its resolution, thissuggest several applications including infrared holography, diffractionstudies and the testing of infrared optical system resolution. It istherefore to. be understood that, within the scope of the appendedclaims, the invention may be practiced otherwise than as specificallydescribed.

What is claimed is:

1. An infrared imaging device for detecting and measuring irradianceswithin a predetermined range of values in a laser beam comprisingcholesteric liquid crystal means to enable visual observation of saidirradiances;

means for cooling said liquid crystal means to permit extended operationof the imaging device over high power average irradiances;

means for preventing infrared radiation from passing from said liquidcrystal means to said cooling means; and

means for supporting and holding said liquid crystal means and saidcooling means in a cooperatin operative relationship.

2. A device as recited in claim 1 in which the cholesteric liquidcrystal means comprises a layer of cholesteric liquid crystals appliedto a supporting medi- 3. A device as recited in claim 2 in which thesupporting medium comprises a sheet of mylar film.

4. A device as recited in claim 3 in which the mylar films thickness isin the range of from 0.0005 to 0.005 inch.

5. A device as recited in claim 2 in which the cooling means comprises areservoir of circulating coolingwater of sufficient volume so as toenable extended operation of the imaging device over relatively highpower average irradiances, said reservoir having means for raising,lowering and maintaining the coolingwater at a desired temperature.

6. A device as recited in claim 5 in which the supporting and holdingmeans comprises a housing segculating through said cavity.

8. A device as recited in claim 7 in which said water pressure headmaintaining means and turbulance region causing means comprise arotatable impellor positioned in said cavity.

9. A device as recited in claim 2 in which said infrared radiationprevention means comprises a reflective coating of aluminum affixed tothe opposite surface of said supporting medium'from that surface to 0which said layer of cholesteric liquid crystals is applied.

2. A device as recited in claim 1 in which the cholesteric liquidcrystal means comprises a layer of cholesteric liquid crystals appliedto a supporting medium.
 3. A device as recited in claim 2 in which thesupporting medium comprises a sheet of mylar film.
 4. A device asrecited in claim 3 in which the mylar film''s thickness is in the rangeof from 0.0005 to 0.005 inch.
 5. A device as recited in claim 2 in whichthe cooling means comprises a reservoir of circulating cooling-water ofsufficient volume so as to enable extended operation of the imagingdevice over relatively high power average irradiances, said reservoirhaving means for raising, lowering and maintaining the cooling-water ata desired temperature.
 6. A device as recited in claim 5 in which thesupporting and holding means comprises a housing segmented into pluralcompartments, the first of said compartments being capable of holdingsaid water reservoir and another of said compartments being a cavitypositioned between said supporting medium and said first compartment. 7.A device as recited in claim 6 further comprising means for maintaininga water pressure head in said cavity and means for causing a turbulanceregion to exist in said cavity adjacent to said supporting medium toexpedite heat transfer from said liquid crystal layer and saidsupporting medium to said cooling-water circulating through said cavity.8. A device as recited in claim 7 in which said water pressure headmaintaining means and turbulance region causing means comprise arotatable impellor positioned in said cavity.
 9. A device as recited inclaim 2 in which said infrared radiation prevention means comprises areflective coating of aluminum affixed to the opposite surface of saidsupporting medium from that surface to which said layer of cholestericliquid crystals is applied.